Without limiting the scope of the invention, its background is described in connection with electronic device packaging, as an example.
Modern electronic components utilize numerous integrated circuits. These integrated circuits are electrically connected to each other or to other electronic components. One method for connecting integrated circuits to electronic components utilizes an area array electronic package, such as a ball-grid array (BGA) package or a flip-chip package. The electrical connections between an integrated circuit packaged in an array package design and a printed circuit board (PCB) are typically composed of solder. Therefore, an important step in the interconnection of many electronic components is tie formation of solder balls in the area array electronic package.
Heretofore, in this field, solder balls have been typically formed utilizing one of four methods: (1) electroplating; (2) printing of solder paste through a stencil or mask; (3)evaporation; or (4) mechanical transfer of preformed solder balls. Electroplating, printing of solder paste through a stencil or mask, and evaporation techniques have been typically utilized for forming solder bumps on wafers and integrated circuits. On the other hand, BGA and chip-scale packages (CSP) have commonly utilized printing of solder paste and mechanical transfer of solder ball techniques.
Electroplating techniques are very well known in the art. In a typical stencil method, solder paste is applied on top of a stencil with milled apertures. A squeegee is moved across the top of the stencil to force the solder paste to go through the apertures to reach contact pads of the electronic package substrate. Conventional evaporation methods involve the use of a vacuum chuck to transfer preformed solder balls to the substrate.
Another method for transferring preformed solder balls utilizes formation of a pattern of dots onto a photoimageable coating laminated to an organic film. The pattern is formed by placing a photomask on the coating and then exposing the coating to a dose of ultraviolet radiation. For example, for an area array package design, the photomask will contain a mirror image of the contact pads design. The areas protected by the photomask design retain their adhesiveness while the unprotected areas exposed to the ultraviolet radiation lose their adhesiveness. The array of adhesive areas corresponds to the pattern of contact pads found on the substrate to receive the solder connections.
After the adhesive areas are formed, solder particles are loaded onto the surface of the film and attach to the adhesive areas. The excess solder particles that lie on the non-adhesive areas are removed. The populated film is then aligned and brought into contact with contact pads, which may be fluxed. A solder reflow is performed to transfer the solder particles from the adhesive areas to the contact pads of the substrate. Following the reflow cycle, the film is removed from the solder particles and discarded.